build: add explicit --no-show-signature for git

When `log.showSignature` is set, it causes the `SOURCE_DATE_EPOCH` to
include a textual signature description on OpenPGP-signed commits,
because Git prints the description into stdout. This then causes some
scripts to fail because they cannot parse the date from the variable.

Adding an explicit `--no-show-signature` prevents the signatures from
being displayed even when one has Git configured to show them by
default, fixing the scripts.

Signed-off-by: Oto Šťáva <oto.stava@gmail.com>
(cherry picked from commit 1e93208bd2)

scripts/get_source_date_epoch.sh

@@ -15,8 +15,8 @@ try_version() {
 }
 
 try_git() {
-	SOURCE_DATE_EPOCH=$(git -C "$SOURCE" log -1 --format=format:%ct \
-		"$SOURCE" 2>/dev/null)
+	SOURCE_DATE_EPOCH=$(git -C "$SOURCE" log -1 --no-show-signature \
+		--format=format:%ct "$SOURCE" 2>/dev/null)
 	[ -n "$SOURCE_DATE_EPOCH" ]
 }